The invention relates generally to an apparatus for lowering the temperature of a heated object in air or any other gas. More particularly, the invention relates to an apparatus for increasing heat transfer from an object at a temperature above ambient by directing a localized corona discharge toward the heated object. Even more particularly, the invention relates to an apparatus for increasing heat transfer from an object at above ambient temperature by applying a voltage (+DC or AC) of about 5000 volts between a needle and a small conducting screen near the needle point and placing this assembly near the heated object in air or any other gas. If the object of interest is initially at a temperature lower than ambient, the present apparatus may be used to raise the object temperature to ambient.
Heretofore and up to the present time, other methods utilizing an electric field have been evolved for this purpose. The nearest approach to the present apparatus utilizes pointed probes charged to very high voltages (15-60 kilovolts) and directed toward heated samples which are electrically grounded. Such high voltage probes must be located at least several centimeters from the heated objects to prevent spark discharge. The possible disadvantages of these methods are that the high voltages required may constitute a safety hazard, the high voltages required may be beyond the normal output range of readily available high voltage supplies, and adequate electrical grounding of the heated sample is often difficult or impractical, particularly if the sample is electrically nonconducting. In addition, these previous methods do not localize the electric field, and a high field gradient exists at the surface of the heated object.
It is shown that the mechanism for cooling by these methods is a phenomenon usually known as the electric, or corona, wind. A nonuniform electric field, such as produced by a charge needle and grounded plane, generates air motion away from the charged point by several electric forces. The significant feature of the effect is that all physical phenomena responsible for the wind occur within usually a millimeter of the charged point. It is therefore not necessary to impose a very high voltage across an air gap of typically centimeters. Rather, the charged probe can be miniaturized, i.e., made physically smaller, operated at considerably lower voltage, placed in close proximity to the heated sample if desirable, and electrically screened to largely shield the heated object and its environment from the electric field.
The referred to miniaturization requires that the electric field be concentrated along the axis of the probe and in the direction of the object to be cooled. An essential feature of the design disclosed herein is that the proper field distribution is obtained by using a grounded screen which subtends a relatively small angle at the probe tip and which is rigidly supported on the probe axis by a thin cylindrical housing of nonconducting material provided with slots to allow the entrainment of air or other cooling gases. The construction provides for a concentrated directional cooling, that is, the structure is designed to concentrate the amperage or electric current flow in the desired direction, e.g., toward the sample being cooled. Heretofore the direction of current flow has usually been dispersed outward from a probe type over a wide angle. Since under the theory of ionic drag the airflow moves in the direction of current flow the airflow has not been concentrated. With the present invention the sides of the probe are an insulator in the form of a tube, in this case cylindrical and longitudinally slotted, and the screen small. Thus, the current flowing from the probe to the screen is concentrated in the direction of the screen and air currents resulting from ionic drag flowing from the screen to the sample are directionally concentrated toward the sample. If the screen were greatly enlarged or if the probe had screen all around or if no screen is used, air movement is more dispersed and is not the concentrated directional airflow provided by this invention.
It is noted that in the prior art, 15 to 60 kilovolts were required for usable cooling. In this invention, typically 5 kilovolts are needed although higher voltage can be used including those of the prior art. The advantages are smaller, more compact, less expensive power supply and interconnecting cables; less chance for dielectric breakdown (insulation failure), and thus safer to personnel.
Another prior art limitation is the necessity of electrically grounding the sample to be cooled, whereas in the present apparatus the sample need not be grounded. Thus for small, fragile, expensive samples, particularly ones which are not good conductors, it may be difficult to properly ground the sample if the previous method is used. In the present apparatus, nothing need be physically attached or be in contact with the sample. The chance of disturbing a delicate alignment, scratching a sample, or changing its inherent characteristics is thus alleviated.
Another limitation in the prior art arises from the typical utilization of 15 - 60 kilovolts. The probe sample spacing is typically several centimeters. In the present invention, the probe may be located almost arbitrarily near the sample permitting compact design, particularly for confined regions with limited available space.
Yet another limitation in the prior art was the existence of a high electric field at the sample surface. In the present apparatus, the electric field is excluded from the sample. This reduces the possibility of damage to the sample, particularly in the case of an inadvertent spark discharge from the high voltage needle. In the present case, the discharge terminates on the screen, and the sample is protected.